2.0 Analysis 2.1 Pilot Flying It could not be determined who was flying the aircraft. The pilot-in-command was qualified for the flight, and was capable of flying the aircraft even if he was in the right-hand seat. 2.2 Survival Aspects The cockpit structure provided little protection for the occupants in the event of a nose-down impact. The use of shoulder harnesses and helmets probably would have resulted in less severe injuries to the occupants. Also, the pilot in the right-hand seat probably would not have been pinned between the ground and the spar. 2.3 Engine Failure 2.3.1 Engine Cooling System The incorrect installation of the bypass hose and the oversized radiator resulted in a decrease in the temperature of the coolant being returned to the engine. The builder's intention, when installing the cooling system, was to prevent the engine from overheating during manoeuvres on the ground on hot summer days. Nevertheless, the result was that coolant was returned to the engine at a lower temperature, and the temperature differential of the coolant between the engine outlet and the pump return became greater. During operations at very low ambient temperatures, this cooling system, with its increased efficiency, would lower the temperature of the coolant by more than 40 degrees Fahrenheit before the coolant returned to the engine block. The colder liquid returning to the pump and circulating through the jackets around the two cylinders and the cylinder head reduced thermal expansion of the cylinders and cylinder head. 2.3.2 Combustion Temperature The carburettor for each of the two cylinders was fitted with one main jet with a setting of 165 (for warm weather), rather than 175 (for cold weather) as recommended by the engine manufacturer. Consequently, the very low ambient temperature of minus 14 degrees Celsius produced a leaner air/fuel mixture, which resulted in higher combustion temperatures and greater thermal expansion of the pistons. The colour of the spark plugs, piston surfaces, and exhaust manifolds indicated a higher combustion temperature in the rear cylinder than in the front. The cause of the difference in combustion temperatures can be traced to the seal at the rear of the engine block near the reduction gear. Only the bearing adjacent to this seal was damaged and showed signs of overheating. The damage to and overheating of the bearing apparently originated with air being drawn in at each engine cycle, which led to degradation of the lubrication to the bearing. The cause of the seal failure could not be determined due to a lack of evidence. However, bearings rarely deteriorate to that point after only 33 hours of service; this implies a defect in the seal. Air sucked in through the defective seal caused thinning of the air/fuel/oil mixture aspirated into the rear cylinder, producing a higher combustion temperature in the cylinder during take-off. 2.3.3 Loss of Piston/Cylinder Clearance The grooves on the piston and on the rear cylinder indicate that the engine stopped because of a loss of clearance (seizure). The piston seizure was caused in part by increased thermal expansion of the piston due to the higher combustion temperature resulting from air being drawn in through the defective seal. Additionally, the seizure was also caused by reduced thermal expansion of the cylinder due to excessive engine cooling. 2.3.4 Engine Manufacturer Service Bulletins Because of this engine's particular susceptibility to stopping, the engine manufacturer posted a warning on the first page of the manual and issued service bulletins to that effect. The service bulletin on the engine cooling system was issued before the aircraft builder purchased the engine. The builder, therefore, probably did not have all the information regarding the installation of the cooling system and engine operation, because purchasers normally do not receive bulletins issued previous to their date of purchase. 2.4 Aircraft Behaviour During the engine warm-up on the ground, the pilot could not have raised the engine power level to the point where he would have obtained a reliable exhaust gas temperature reading, as the ski-equipped aircraft would have slid on the hard snow-covered surface. During the take-off run, full engine power was applied. The increase in the exhaust gas temperature reading was almost instantaneous, but more time was required for the instruments to show a decrease in coolant temperature. However, during the take-off, it is possible that neither of the occupants observed this situation on the instruments. During the climb-out to 200 feet agl, the rear piston continued to expand, while cooling of the cylinder was more effective; both events contributed to the piston seizure. Before the engine stopped, the aircraft was turning slowly to the right during the initial climb. When the engine stopped, the ultralight pitched nose up and lost speed very quickly. The stall speed for this aircraft is close to the climb speed; the angle of attack of the left wing, which is the outside wing in the turn, exceeded that of the right wing, and the left wing stalled first. The stall occurred at an altitude insufficient for the pilot to regain control of the aircraft and make an emergency landing on the cleared area straight ahead. The aircraft entered a left-hand spin before it struck the ground. 3.0 Conclusions 3.1 Findings The pilot was qualified for the flight. It could not be determined which of the two occupants was flying the aircraft. The stall speed for the Amphibec is close to the climb speed. The ultralight stalled at an altitude insufficient for the pilot to make a recovery and regain control of the aircraft. The cooling system installation was not in accordance with the diagram provided by the cooling system manufacturer, nor did the installation conform with the recommendations of the engine manufacturer. The oversized radiator and ambient temperature of minus 14 degrees Celsius were conducive to excessive cooling of the cylinders. The type 165 (warm weather) carburettor jets and the temperature of minus 14 degrees Celsius caused higher combustion temperatures and increased thermal expansion of the pistons. The engine stopped because of a lack of clearance (seizure) between the rear piston and cylinder. There was no indication that the builder had all the documentation concerning the installation of the cooling system and the operation of the engine. The aircraft was not equipped with shoulder harnesses, and the occupants were not wearing helmets; such equipment was not required by regulations. Only the crankshaft bearing near the propeller reduction gearbox showed signs of overheating and insufficient lubrication. The crankshaft bearing rear seal allowed air to leak in and thin the air/fuel/oil mixture. Only the rear piston lost clearance with the cylinder. 3.2 Causes The engine stopped when the rear piston seized. The aircraft then stalled at an altitude insufficient for a recovery. Factors contributing to the piston seizure were the defective crankshaft seal, the very low ambient temperature, the use of an oversized radiator, incorrect installation of the cooling system, and the use of type 165 (warm weather) carburettor jets. The Board has no aviation safety recommendations to issue at this time.4.0 Safety Action The Board has no aviation safety recommendations to issue at this time.